Methods for designing and producing a device comprising an array of micro-machined elements, and device produced by said methods

ABSTRACT

A design process is used for designing a device comprising a plurality of micro-machined elements, each comprising a flexible membrane, the elements being arranged in a plane in a determined topology. The design process comprises a step of defining the determined topology so that it has a character compatible with a generic substrate having cavities, the characteristics of which are pre-established. Each flexible membrane of the micro-machined elements is associated with one cavity of the generic substrate. The present disclosure also relates to a fabrication process for fabricating a device comprising a plurality of micro-machined elements, and to this device itself, wherein only some of the pairs of cavities and flexible membranes are configured to form a set of functional micro-machined elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/963,924, filed Jul. 22, 2020, which is a national phase entry under35 U.S.C. § 371 of International Patent Application PCT/FR2019/050108,filed Jan. 18, 2019, designating the United States of America andpublished as International Patent Publication WO 2019/141952 A1 on Jul.25, 2019, which claims the benefit under Article 8 of the PatentCooperation Treaty to French Patent Application Serial No. 1850484,filed Jan. 22, 2018.

TECHNICAL FIELD

The present disclosure relates to devices comprising an array ofmicro-machined elements arranged in a plane. More particularly, thepresent disclosure relates to an acoustic device implementing an arrayof micro-machined ultrasonic transducers. These transducers may be ofpiezoelectric or capacitive nature. The present disclosure is, forexample, applicable to the field of ultrasound imaging.

BACKGROUND

The document (D1) Lu, Yipeng, and David A. Horsley, “Modeling,fabrication, and characterization of piezoelectric micro-machinedultrasonic transducer arrays based on cavity SOI wafers,” Journal ofMicroelectromechanical Systems 24.4 (2015): 1142-1149 recalls thatmicro-machined ultrasonic transducers (MUTs) are formed from a membraneoverhanging a cavity, the membrane having a low acoustic impedance and agood coupling to air or another fluid through which an acoustic wavepropagates. In receiver mode, the bowing of the membrane under theeffect of the acoustic wave is converted into an electrical signal. Inemitter mode, an electrical signal makes the membrane bow in order togenerate an acoustic wave.

Documents US20130270967, US20140219063 and US20170128983 provide otherillustrations of MUT devices, and fabrication processes thereof.

MUT technology allows acoustic devices, which are, for example,applicable to the imaging field, to be designed and has many advantages,in particular, its ease of integration with signal-conditioningelectronics, its low power consumption, a wide passband and the easewith which extensive arrays of transducers arranged in a plane may befabricated therein.

MUT transducers may be classified into two categories depending on themechanism of activation of the membrane.

Capacitive MUTs, a cross section of one of which has been schematicallyshown in FIG. 1 a , have two electrodes 1 a, 1 b, one arranged on themembrane 2, the other generally being formed by the bottom of the cavity3 that the membrane overhangs. This second electrode may thereforeconsist in a single contact 1 b made on the carrier 4 on which thecavity 3 rests. These two electrodes form a capacitor, the capacitanceof which depends on the degree of flexion of the membrane.

Piezoelectric MUTs, a cross section of one of which has beenschematically shown in FIG. 1B, also have two electrodes 1 a, 1 b, whichare arranged on the membrane 2, on either side of a layer 5 ofpiezoelectric materials. In this type of device, the layer 5 ofpiezoelectric materials is coupled to the membrane 2 order to make itbow or to track its bowing.

Whatever the type of MUTs that an acoustic device employs, the MUTs aregenerally arranged in a plane and in a determined topology, as is wellillustrated in the document (D2) Ergun, Arif S., Goksen G. Yaralioglu,and Butrus T. Khuri-Yakub, “Capacitive micro-machined ultrasonictransducers: Theory and technology,” Journal of aerospace engineering16.2 (2003): 76-84.

By “topology” what is meant, in the context of the present disclosure,is the geometry of each transducer and, in particular, the shape and thedimensions (width, length or diameter, depth) of the cavity and of theassociated membrane that it comprises, and the distribution in the planeof the transducers forming the acoustic device.

An example topology comprising a plurality of blocks b1, b2 oftransducers, the blocks being separated from one another by a blockseparation distance p2 (measured between the center of two transducersarranged facing on opposite edges of two blocks b1, b2) is shown, veryschematically, in FIG. 1 c . A device may comprise one such block, orindeed several hundred such blocks separated from one another by adistance p2, for example, of about 10 to 100 microns. Contacts, tracksor other elements allowing, for example, the transducers to be connectedto a conditioning circuit are generally arranged in the space availablebetween each block. Each block has a determined width and length (whichmay be expressed in number of transducers in each of these dimensions)and delineates a 2D array of transducers t that are generally regularlydistributed. One block may comprise several thousand transducers. Eachtransducer is separated from a neighboring transducer by a determinedseparation distance p1, typically comprised between one micron and 100microns. Each transducer comprises a membrane overhanging a cavity thathas, in the shown example, a circular shape and a diameter d (of a fewmicrons to a few hundred microns). The topology is also defined by thedepth of the cavities, which may be between one micron or less andseveral tens of microns or more.

The design of an acoustic device generally requires the topology thatwill be employed to be precisely defined, i.e., the number anddimensions of the blocks of transducers, the block separation distance,the arrangement of the transducers within a block (for example, in theform of a matrix array, the separation distance between two transducersbeing specified) the dimensions and shape of the cavities of eachtransducer, etc., to be precisely defined. These design parameters aredictated, to a certain extent, by the expected performance and thefunctionality of the acoustic device. It is not necessary for all thetransducers or all the blocks of transducers of an acoustic device toall have the same dimensions or one single layout.

Document D1 also recalls that there are several ways of fabricating anacoustic device based on MUTs, depending on whether the transducers arefabricated at the same time, after, or separately to the analogue anddigital signal-conditioning electrical circuits. This document thusproposes a simplified process for fabricating an acoustic device inwhich the transducers are fabricated separately from the conditioningcircuits. This process takes advantage of cavities formed beforehand ina substrate to limit the number of photolithography mask levels needingto be employed.

This approach is attractive, but it means that some of the effort madedesigning the acoustic device must be borne by the manufacturer of thesubstrate. Specifically, the manufacturer of the substrate must be ableto design and fabricate a substrate containing cavities having exactlythe topology chosen for the device. However, device topology is often apiece of valuable and sensitive information that the device manufacturergenerally does not want to divulge.

In addition, the substrate manufacturer must develop substratefabrication processes specific to each topology. The effort required todo this makes rapid and economical development of acoustic devices moredifficult.

It would therefore be desirable to be able to provide a substratecontaining cavities that would be able to serve in the fabrication of avast range of acoustic devices and that would therefore not require adhoc development, for each acoustic device, of a specific substratefabrication process.

BRIEF SUMMARY

With a view to achieving one of these aims, one subject of the presentdisclosure is a design process for designing a device comprising aplurality of micro-machined elements each comprising a flexiblemembrane, the elements being arranged in a plane in a determinedtopology. This design process comprises a step of defining thedetermined topology so that it has a character compatible with a genericsubstrate having cavities the characteristics of which arepre-established, each flexible membrane of the micro-machined elementsbeing associated with one cavity of the generic substrate.

With a view to achieving one of these aims, one subject of the presentdisclosure is a design process for designing a device comprising aplurality of micro-machined elements each comprising a flexiblemembrane, the elements being arranged in a plane in a determinedtopology. This design process comprises a step of defining thedetermined topology so that it has a character compatible with a genericsubstrate having cavities the characteristics of which arepre-established, each flexible membrane of the micro-machined elementsbeing associated with one cavity of the generic substrate.

Thus, a design rule is defined that sets the topology of the device sothat its membranes are in register with the cavities of the genericsubstrate. It is therefore not necessary to develop one particularsubstrate designed for the topology of the device.

According to other advantageous and non-limiting features of the presentdisclosure, which may be implemented alone or in any technicallyfeasible combination:

-   -   in the step of defining the topology, the generic substrate is        chosen from a group of generic substrates, the substrates having        pre-established characteristics that differentiate them from one        another;    -   the defining step comprises selecting the generic substrate from        the group of generic substrates, such that the determined        topology is defined so as to be as similar as possible to a        desired topology;    -   the design process also comprises defining the thicknesses of        the membranes;    -   the step of defining the determined topology involves:        -   choosing the dimensions and shape of the membranes of the            elements so that the dimensions and shape correspond to the            shape of the cavities of the generic substrate;        -   choosing the arrangement of the membranes of the            micro-machined elements in the plane so that they are all            plumb with a cavity of the generic substrate when the device            is in register with this substrate;    -   the arrangement of the cavities of the generic substrate is        regular and covers all its extent;    -   the micro-machined elements are micro-mirrors or micro-machined        acoustic or ultrasonic transducers;    -   the generic substrate has cavities the sizes and/or shapes of        which are not all identical.

According to another aspect, the present disclosure proposes afabrication process for fabricating the designed acoustic device, whichcomprises a step of providing the generic substrate.

More generally, the present disclosure proposes a fabrication processfor fabricating a device comprising a plurality of micro-machinedelements, the process comprising:

-   -   a step of providing a generic substrate comprising a surface        layer arranged on a carrier, the main face of the carrier having        emergent cavities and the portions of the surface layer        overhanging the cavities forming flexible membranes associated        with the cavities;    -   at least one step of processing only certain of the pairs formed        from one cavity and from one membrane in order to form at least        one functional micro-machined element and at least one leftover        pair that is not able to convert a movement of its membrane into        an electrical signal or vice versa.

According to other advantageous and non-limiting features of thisfabrication process, which may be implemented alone or in anytechnically feasible combination:

-   -   the processing step also comprises removing a membrane and/or a        cavity;    -   the processing step also comprises a step of neutralizing a        membrane and/or a cavity;    -   at least one processing step comprises a step of producing an        array of electrically conductive elements that is configured to        connect only some of the cavity-membrane pairs;    -   the production of the array of electrically conductive elements        comprises forming electrodes plumb with only some of the        cavities;    -   the process comprises a step of adjusting the thickness of the        flexible membranes;    -   the generic substrate comprises an intermediate layer between        the main face of the carrier and the surface layer;    -   the process comprises a step of depositing at least one first        electrode on the surface layer;    -   the process comprises a step of depositing a piezoelectric layer        on at least the first electrode;    -   the process comprises a step of depositing a second electrode on        the piezoelectric layer.

Lastly, the present disclosure proposes a device comprising a carrierand a surface layer arranged on the carrier, the main face of thecarrier having emergent cavities and the portions of the surface layeroverhanging the cavities forming flexible membranes associated with thecavities, only certain of the pairs formed from one cavity and from oneflexible membrane being configured to form a set of functionalmicro-machined elements, at least one leftover pair not being able toconvert a movement of its membrane into an electrical signal or viceversa.

According to other advantageous and non-limiting features of the device,which may be implemented alone or in any technically feasiblecombination:

-   -   the device comprises an array of electrically conductive        elements that is configured to connect only some of the        cavity-membrane pairs;    -   the micro-machined elements are micro-mirrors or micro-machined        acoustic or ultrasonic transducers.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present disclosure will becomeapparent from the following detailed description of example embodimentsof the present disclosure, which description is given with reference tothe appended figures, wherein:

FIGS. 1 a to 1 c show a capacitive MUT, a piezoelectric MUT, and atopology of a prior-art acoustic device, respectively;

FIGS. 2 a to 2 c schematically show a cross-sectional view and top viewsof an example of a generic substrate according to the presentdisclosure.

DETAILED DESCRIPTION

Very generally, the present disclosure on the one hand proposes ageneric substrate (or a group of such generic substrates) havingcavities overhung by membranes. On the other hand it aims to definedesign rules for a device, in particular, an acoustic device, so that itmay be fabricated from such a generic substrate. At least some of themembranes and of the cavities of the generic substrate are intended toform functional micro-machined elements, such as the transducers of anacoustic device. Thus, the substrate manufacturer is able to provide, inbulk and at less cost, substrates without having to adjust thefabrication process to a specific customer requirement. A devicemanufacturer, in particular, a manufacturer of acoustic devices, whorespects these design rules may stock up with substrates without sharingthe often confidential characteristics of their product.

By “cavity,” what is meant in the context of the present disclosure isany void formed in a substrate. It is not necessary for this void to beentirely closed or hermetically closed.

By “membrane” or “flexible membrane,” what is meant is any structuresuspended above a void in the substrate. It may have any shape, and, forexample, form a partition or a beam. The perimeter of the suspendedstructure may be entirely or partially joined to the substrate. It mayalso be joined to the substrate via one or more feet. Whatever the wayin which the structure is joined to the substrate, some at least of thisstructure has at least one degree of freedom of movement, making itflexible.

Generic Substrate

According to one aspect, the present disclosure therefore proposes todefine a generic substrate 20 comprising cavities. Such a genericsubstrate 20, one example of which has been schematically shown in crosssection in the FIG. 2 a and as seen from above in FIG. 2 b , maycomprise a carrier 21 and a surface layer 23 arranged on the carrier. Itmay also comprise an intermediate layer 22 between a main face of thecarrier and the surface layer. The carrier 21 comprises emergentcavities 24 that open onto its main face. The portions of the surfacelayer 23 overhanging the cavities 24 of the carrier 21 form a flexiblemembrane.

The generic substrate 20 may be very easily fabricated using a “bondingand thinning” type process. Such a process comprises providing thecarrier 21 and forming, in the main face of this carrier 21, cavities24, for example, by wet or dry etching, using, if necessary, aphotolithography step to pattern a resist deposited beforehand on themain face of the carrier 21, so as to mask zones to be protected fromthe etching or to open zones to be etched.

The process may also comprise providing a donor substrate and bonding itto the carrier 21 so as to cover the emergent cavities 24. The carrier21 and/or the donor substrate may have been equipped beforehand withlayers, for example, dielectric layers, on the faces thereof that areintended to be bonded to form the buried intermediate layer 22 of thesubstrate 20. The one or more layers formed on the carrier 21 may bedeposited before or after the emergent cavities 24 have been etched intothe surface of this carrier. They may therefore, according to onevariant, coat the walls defining these cavities.

In a following step, the donor substrate is thinned, this possibly beingdone mechanically, chemically or by fracture level with a weakened planethat is formed beforehand in the donor substrate, for example, byimplanting gaseous species such as helium and/or hydrogen. The surfacelayer 23 may have a thickness comprised between 100 nm and several tensof microns, or even several hundred microns.

In certain cases, the bonding step may be carried out in a bondingchamber the atmosphere of which is controlled, for example, in order tohave a determined vacuum pressure and/or a particular gaseous nature.Thus, the atmosphere present in the cavity, and, in particular, itsvacuum level and/or the nature of the gas filling it, is controlled.

Preferably, the bonding step employs molecular bonding i.e., the cleanand smooth bonded surfaces of the carrier 21 and of the donor substrateadhere to each other via Van der Waals or covalent forces, withoutrequiring the application of an adhesive. The forces of adhesion may bestrengthened via a heat treatment the temperature of which may bebetween 100° C. and 1000° C. or more, or promoted by a plasma orchemical treatment of the surfaces required to make contact, before theassembly thereof.

Variants of this process may be envisioned. By way of example, and as isknown per se, the cavities 24 may be formed subsequently to the step ofbonding the donor substrate and the carrier, and to the step of thinningthe donor substrate. One commonplace way of proceeding in the field offabrication of micro-machined devices is based on the use ofsilicon-on-insulator (SOI) substrates into which these cavities 24 areetched. Advantage is taken of the properties whereby the silicon andburied insulator can be etched selectively, to produce the cavities 24in the buried intermediate layer 22 of the SOI substrate. Optionally,the apertures that allowed the cavities 24 to be selectively etched maybe plugged after their formation, in order, in particular, to make themseal tight.

Advantageously, for reasons of availability and cost, the carrier 21 andthe donor substrate consist of silicon wafers. They may thus be circularwafers, the diameter of which may be 200 mm, 300 mm or more. However,they may equally well be wafers of other materials, or be any materialin a form other than a wafer. The material of the donor substrate willpossibly, in particular, be chosen so that the surface layer 23, and, inparticular, the portions of this layer overhanging the cavities 24 andforming membranes, has a determined stiffness, allowing the bowing ofthese membranes when they are placed under stress to be controlled. Inthe same way, provision will possibly be made to choose the nature ofany intermediate layer 22 so as to modify the stiffness of themembranes. It is also possible to choose to form the intermediate layerfrom a dielectric (silicon dioxide or silicon nitride, for example) inorder to electrically insulate the surface layer 23 from the carrier 21.

Provision may be made for alignment marks to be arranged in the surfacelayer 23 or on the sides of the generic substrate 20, so as to make itpossible to know the location of the cavities with precision, and toallow electrodes and other layers required to complete the fabricationof transducers, or more generally of micro-machined elements, to beplaced in perfect alignment with these cavities 24 and thus the elementsto be made functional.

The substrate 20 has a generic character in that the shape, thedimensions, and the arrangement of the cavities over the extent of thesubstrate are not specific to one particular product or to oneparticular application.

Thus, and as is shown in the example of FIG. 2 b , the generic substratemay comprise a plurality of cavities 24 that are uniformly distributedover the entire extent of the substrate 20 and that all have the sameshape and the same dimensions. The expression “the entire extent of thesubstrate” is understood to mean at least one central area of the mainface of the substrate, this possibly excluding a zone peripheral to thisarea. In this example, all the cavities have a circular shape in theplane of the substrate, are of diameter equal to 40 μm, are of 0.2 μmdepth and are uniformly distributed at the intersections of a grid, thegeneratrices of which are perpendicular to one another. The distanceseparating the center of a cavity from the center of an adjacent othercavity is equal to 100 μm.

FIG. 2 c shows a top view of a second example of a generic substrate 20.In this second example, the cavities 24 a, 24 b, 24 c are arranged ingroups, the groups of cavities being uniformly distributed over thesurface of the substrate 20. Each group of cavities comprises, in theshown example, three cavities 24 a, 24 b, 24 c of circular shape thatare arranged in proximity to one another, and that have a diameter of 20μm, 40 μm and 60 μm, respectively. The other parameters of the substrate20 are the same as those of the first example.

More generally, a generic substrate may have a plurality of groups ofcavities, each group of cavities having different characteristics(shape, dimensions, etc.). The groups of cavities may be arranged inseparate zones of the surface of the substrate or, in contrast, thecavities of each group may be arranged side-by-side one another. Eachgroup of cavities may be intended to form one micro-machined elementhaving a determined function. By way of example, a first group ofcavities may have cavity characteristics that make the cavities thereofparticularly suitable for forming emitting transducers, and a secondgroup of cavities may have cavity characteristics that make the cavitiesthereof particularly suitable for forming receiving transducers. In asecond example embodiment, a first group of cavities may have cavitycharacteristics that make the cavities thereof particularly suitable forforming low-frequency transducers, and a second group of cavities mayhave cavity characteristics that make the cavities thereof particularlysuitable for forming high-frequency transducers.

It is possible to envision, in the context of the present disclosure,providing a plurality of types of generic substrate 20. For example,provision may be made to create a collection of types of genericsubstrate modelled on the example of FIGS. 2 a and 2 b , and for whichthe diameter of the cavities 24 is chosen to be 5 μm, 20 μm, 40 μm and60 μm, respectively.

More generally, the types of substrate of the collection havepre-established characteristics that differentiate them from oneanother.

Each of the types of generic substrate 20 requires a specificfabrication process that must be adjusted to take into account the sizeof the cavities 24 and the relative extent of the areas that are placedin contact. However, since the number of generic substrates from whichthe collection is formed is limited, it is relatively easy to definethese processes beforehand and to implement them depending on which typeof substrate is required.

Of course, the present disclosure is in no way limited to the just givenexample of a generic substrate 20. It is possible to envision cavitieshaving a different shape, for example, a square, rectangular or evenhexagonal shape, or a different depth or a different arrangement on thesurface of the carrier. It is, in particular, not necessary for thisarrangement to be regular or for it to cover the entire extent of thegeneric substrate, although this is particularly advantageous and allowsconstraints to be limited during the design of a device.

In any case, the present disclosure makes provision for a limited number(for example, 1, 3, 10 or 20) of types of generic substrate to beestablished, the characteristics of which are precisely predefined.

Design process for designing an acoustic device.

According to another aspect, the present disclosure proposes a designprocess for designing a device comprising micro-machined elements. Inthe example the description of which follows, the functionalmicro-machined elements are micro-machined ultrasonic transducers, andthe device is an acoustic device. This process aims to establish a modelof the device so that it may be subsequently fabricated on a genericsubstrate. Complementarily, it may also establish the various steps thatmust be implemented to fabricate the acoustic device from the genericsubstrate. Of course, the design process will take into account thechosen, capacitive or piezoelectric, nature of the transducers.

As has been seen, such a transducer comprises a flexible membraneoverhanging a cavity, which may be hermetically closed and contain acontrolled atmosphere, and two electrodes placed on either side of themembrane and/or of the cavity. Moreover, the electrodes of thetransducers may be arranged otherwise. Specifically, a group of pairsthat consist of one cavity/one associated flexible membrane, that aredistributed side-by-side and that form one or more transducers, may beaddressed by a single pair of electrodes placed at the ends of thegroup. It is also possible to envision arranging these electrodesdifferently, for example, to interdigitate them on a given face of themembrane, in particular, when the latter comprises a piezoelectricmaterial. The transducers are all distributed in one plane and in adetermined topology. For the sake of clarity, the shape and dimensions,in the plane, of a transducer will be considered to precisely correspondto the shape and dimensions of the membrane forming this transducer.

The design of the acoustic device comprises a step in which thedetermined topology of a plurality of transducers, i.e., of a pluralityof micro-machined elements each comprising one flexible membrane, isdefined. It may thus be a question of defining, depending on thetargeted field of application and on the level of performance expectedfrom the device, the number and the dimensions of the blocks oftransducers, the distribution of the transducers in each block, thedistances separating two adjacent blocks, and the shape and thedimensions of each transducer in the interior of each block.

According to the present disclosure, this defining step is carried outso that the defined topology has a character compatible with a genericsubstrate, i.e., a topology in which the flexible membranes of themicro-machined elements may be associated with the cavities of thegeneric substrate. In other words, this defining step requires thechosen topology to respect a set of design rules ensuring thecompatibility of the topology with a generic substrate. It is thusguaranteed that, at the end of the process, the designed acoustic devicemay be fabricated from such a generic substrate.

Thus, care will be taken, in this step, to ensure that the shape and thedimensions (diameter, length, width, and/or depth) of the membranes ofthe transducers indeed correspond to the shape and to the dimensions ofthe cavities of a generic substrate.

Care will also be taken to ensure that the arrangement of thetransducers in the plane of the topology is such that, projected ontothe surface of a generic substrate, each transducer may be positionedplumb with a cavity. It is not however necessary for all the cavities ofa generic substrate to correspond to one transducer, certain cavities ofthe generic substrate possibly not being employed in the acousticdevice.

Thus, when a generic substrate comprises grouped-together cavities ofvarious dimensions, as is the case in the second example of a genericsubstrate, which example is illustrated in FIG. 2 c , the topology ofthe acoustic device will possibly be defined so that only the cavitiesof one particular dimension are used. Alternatively, the plurality ofgroups of cavities will possibly be used to form transducers havingdifferent properties, as was described in the aforementioned example.

By way of example, the latter rule may lead to the following beingrequired:

-   -   in each block of transducers, for the transducers to be        uniformly distributed in the plane, and to be arranged at the        nodes of a grid the generatrices of which are perpendicular to        one another;    -   for the distance separating two adjacent blocks to correspond to        a multiple of the distance separating two transducers.

Thus, it is guaranteed that this distribution is not incompatible with ageneric substrate of the type described with reference to FIGS. 2 a-2 c, i.e., a substrate the cavities of which are uniformly distributed inthe plane of the substrate.

The design process is of course not limited to this topology-definingstep. It comprises other steps such as the definition of the thicknessof the membranes, the definition of the geometry and the routing of theelectrodes in each block, the definition of zones of electrical contactaround these blocks, and any other design steps that are conventionallyimplemented to make the micro-machined elements functional and toproduce a complete model of an acoustic device.

The determined topology may differ from a desired topology, which couldstem directly from functional analysis of the device that is in theprocess of being designed. For example, the dimensions and shape thatthe cavities of the transducers are required to have for these cavitiesto correspond to the shape and dimensions of the cavities of the genericsubstrate may lead to a discrepancy between the resonant frequency ofthe membrane and the frequency that was desired in the first steps ofthe design process.

To limit the impact of this discrepancy, a process according to thepresent disclosure may comprise a step of adjusting certain parametersof the acoustic device that is in the process of being designed. Ofcourse, this adjusting step can concern only parameters that do not callinto question the compatibility of the topology with the genericsubstrate. A process according to the present disclosure may also makeprovision for the introduction, into the process for fabricating theacoustic device, of complementary steps for adjusting certain parametersof the device during its fabrication.

By way of example, it is possible to envision modifying the thickness ofthe membrane during the fabrication of the device and to thus adjust theresonant frequency of a transducer. It is thus possible to compensatefor any frequency discrepancy related to the cavity shapes and/ordimensions that had to be used because of the generic substrate. Thisadjustment, by thickening or thinning, may be made to the membrane ofone particular transducer, to one group of transducers, or to all thetransducers of the acoustic device. The material of the thickening layermay be chosen for its effect on the dynamic behavior of the membrane.

To limit the impact of discrepancies, provision will also possibly bemade to provide a sufficient number of different generic substrates,well distributed over the design-choice space. In this case, there maybe a generic substrate that, for any desired topology, requires adetermined topology that is sufficiently close to the desired topologyfor the impact on the performance and functionality of the acousticdevice to be minimal.

The design process for designing an acoustic device is naturallyimplemented by computational means. These means may be configured tofacilitate the definition of the determined topology, for example, byautomatically determining the determined topology from a desiredtopology. To do this, the process must have access, via availablecomputational means, to the preset characteristics of the one or moregeneric substrates.

In the case where a collection or a group of generic substrates isavailable, the process may search for and choose the most suitablegeneric substrate of the group and propose the determined topology thatis the most similar to the desired topology, i.e., the topology thataffects the functionality and/or performance of the device the least.

Fabrication process for fabricating the acoustic device.

According to another aspect, the present disclosure also relates to afabrication process for fabricating an acoustic device, and to a devicefabricated using this process. As was noted above, at least certainsteps of this fabrication process may have been defined, at least asregards their main parameters, during the design process itself.

The fabrication process comprises providing an example of the genericsubstrate that, in the design process, was selected from the group ofgeneric substrates. Usually, a plurality of devices may be fabricated ona generic substrate using wafer-scale processing. It will be recalledthat the generic substrate has cavities the characteristics of which arepre-established, the cavities, respectively, being associated withflexible membranes that overhang them.

According to this aspect of the present description, the fabricationprocess comprises at least one processing step in which only certain ofthe pairs formed from one membrane and from one cavity are activated ordeactivated in order to form at least one functional micro-machinedelement and at least one leftover pair that is non-functional, i.e.,that is not able to convert a movement of its membrane into anelectrical signal or vice versa.

Certain of these processing steps are carried out only on thosemembranes of the generic substrate that are specified, in the topology,as being required to form a transducer. As was seen above, it istherefore not necessary for the processing steps to lead to transducersbeing formed from all the cavities present in the generic substrate.Thus, with reference once again to FIG. 1 c , the zones separating twoadjacent blocks of transducers will not be processed in certain of theseprocessing steps, although they overhang cavities of the genericsubstrate.

In other words, in the fabrication process, certain of the pairs formedfrom one cavity and from one membrane of the generic substrate areactivated to form functional micro-machined elements, i.e., elementsable to reciprocally convert a movement of the membrane into anelectrical signal. The one or more inactivated or deactivated leftovercavity-membrane pairs will therefore not be used by the acoustic device.

Thus, in one embodiment, the leftover cavity-membrane pairs of thegeneric substrate, i.e., pairs not selected in the design phase, areremoved. This removal may be achieved via a processing step in which theassociated cavity-membrane assemblies are completely or partially etchedaway.

In another embodiment, the leftover cavity-membrane pairs of the genericsubstrate, i.e., pairs not selected in the design phase, areneutralized, without necessarily causing any trace thereof to disappear.Such a neutralization may be achieved via a processing step and takemultiple forms: the flexible membrane may be blocked, the routing ofinterconnects may be interrupted, the operating points with respect to aresonant frequency may be shifted, or the membrane may even be pierced,etc.

One processing step may also comprise a step of adjusting the thicknessof the surface layer, in order to adjust the thickness of the membranesoverhanging the cavities. The adjusted thickness of the membranes may beobtained by thickening or thinning the surface layer. This thinning orthickening may be carried out locally, on the membrane of one particulartransducer or on one group of transducers, or on all the membranes ofthe device, using conventional photolithography techniques allowing thezones to be thinned and/or thickened to be defined. Thinning ispreferably achieved via an etching step: chemical etching in solution,gaseous or plasma chemical etching, sacrificial oxidation, or physicaletching by ion-beam sputtering. Thickening is preferably achieved viadeposition of the same material as that initially forming the membrane,in general silicon. The deposited layer may be single-crystal when it isa question of epitaxy. However, it may also be polycrystalline oramorphous. It is also possible to make use of other materials so as toproduce, by way of end result, heterogeneous suspended structures: forexample, a deposition of SiO₂, Si₃N₄ or of metal on an initial membranemade of single-crystal silicon. Thickening may also be achieved byoxidation, by converting some of the thickness of the original siliconfilm into SiO₂, which oxidation is accompanied by an effect that swellsthe layer, which effect is well known and well characterized by thoseskilled in the art. Other variations in and adjustments of thickness areliable to occur in several steps of the production process forfabricating the acoustic device: for example, during the formation of apiezoelectric layer, or even the production of the electrodes.

A processing step may also comprise a step of producing an array ofelectrically conductive elements configured to electrically connectcertain of the cavity-membrane pairs and to make the correspondingmicro-machined elements functional. The production of the array ofelectrically conductive elements may comprise forming electrodes, inparticular, plumb with only some of the cavities.

Depending on whether the employed transducers are capacitive orpiezoelectric, the fabrication process will comprise other processingsteps required to complete the fabrication of the transducers.

It may thus comprise a step of depositing at least one first electrodeon the surface layer, and in the case of a piezoelectric transducer, astep of depositing a piezoelectric layer on the first electrode, and astep of depositing a second electrode on the piezoelectric layer.

In the case of a capacitive transducer, it may be necessary to producean aperture in order to allow the second electrode to be placed on thecarrier.

At the end of the fabrication process, an acoustic device comprising acarrier and a surface layer placed on the main face of the carrier isobtained. The carrier has emergent cavities and the portions of thesurface layer overhanging the cavities form flexible membranes that areassociated with the cavities. Only some of the pairs formed from onecavity and from one flexible membrane of the generic substrate areconfigured to form a set of functional micro-machined elements, i.e.,elements able to reciprocally convert a movement of the membrane into anelectrical signal.

As was seen above, the cavities and/or the membranes of the genericsubstrate that are not activated, for example, those arranged in thezones separating two blocks of transducers, may be preserved,neutralized or removed, for example, by wet or dry etching of thesurface layer, of the intermediate layer if it is present, or even ofone portion of the carrier. Thus, flexible membranes that are notactivated and the cavities that they overhang are removed.

It is also possible to use certain unemployed zones of the substrate,which nonetheless comprise cavities, as streets for the dicing of thesubstrate to singulate the devices fabricated by wafer-scale processingof the generic substrate.

Of course, the present disclosure is not limited to the describedembodiments and examples and variant embodiments may be renderedtherefrom without departing from the scope of the present disclosuresuch as defined by the claims.

The present disclosure, is, in particular, not limited to the design ormanufacture of acoustic devices comprising micro-machined ultrasonictransducers.

It may, for example, be a question of transducers able to operate infrequency ranges other than the ultrasound frequency range. It is thuspossible to envision forming loudspeakers or microphone arrays. Eachtransducer may then be addressed individually or with packages, thisbeing particularly advantageous in the field of high-fidelity acousticsor directional acoustics.

The present disclosure is more generally applicable to any devicecomprising a plurality of micro-machined elements employing membranes orother suspended structures that are arranged in an array in a plane andin a particular topology, and that are able to be processed to formfunctional micro-machined elements.

It may thus, for example, be a question of micro-machined elementsdefining reflective zones with a view to forming arrays of controllablemovable micro-mirrors. Such micro-systems may be produced with a view toforming light beams, such systems enabling new projecting systems,screens and remote-recognition systems. In this case, the flexiblemembranes of the generic substrates form the carrier of themicro-mirrors, which become movable and controllable after they havebeen functionalized in the topology determined in the design phase. Thecavities in this case correspond to the void produced in the substratein order to make it possible for the micro-mirrors to see-saw. Themicro-mirrors may be anchored in several ways: via the entirety or someof their perimeter, via a single central foot or in contrast viamultiple and distributed feet. The activation or in contrast thedeactivation of only some of the micro-mirrors may be achieved in manyways: simple removal by selective etching; releasing or in contrastblocking the micro-mirrors by removing or forming blocking points,respectively; forming or in contrast removing a reflective layer fromthe micro-mirrors; forming or in contrast removing an absorbing layerfrom the micro-mirrors.

1. A device, comprising: a substrate including a carrier and a surfacelayer arranged on the carrier, a main face of the carrier havingemergent cavities, portions of the surface layer overhanging thecavities forming flexible membranes associated with the cavities, thesurface layer and the main face of the carrier being bonded by molecularbonding, only some pairs of cavities each comprising one cavity and fromone flexible membrane being configured to form a set of functional pairsof micro-machined elements, leftover non-functional pairs not being ableto convert a movement of its membrane into an electrical signal or viceversa, the functional pairs and non-functional pairs distributed overthe entire extent of the substrate.
 2. The device of claim 1, furthercomprising an array of electrically conductive elements that isconfigured to connect only some of the cavity-membrane pairs.
 3. Thedevice of claim 1, wherein the micro-machined elements aremicro-mirrors, or micro-machined acoustic or ultrasonic transducers. 4.The device of claim 1, further comprising an intermediate layer betweenthe main face of the carrier and the surface layer.
 5. The device ofclaim 4, wherein the intermediate layer comprises a dielectric material.6. The device of claim 5, wherein the dielectric material comprises atleast one of silicon dioxide or silicon nitride.
 7. The device of claim1, further comprising a dielectric coating on walls of the carrierdefining the emergent cavities.
 8. The device of claim 1, wherein thesurface layer has a thickness between 100 nm and several hundredmicrons.
 9. The device of claim 1, wherein the device is a genericsubstrate, and a shape, dimensions, and an arrangement of the emergentcavities over an extent of the generic substrate are not specific to oneparticular product or to one particular application.
 10. The device ofclaim 9, wherein the emergent cavities are uniformly distributed over anentire extent of the substrate and have the same shape and the samedimensions.
 11. The device of claim 9, wherein the emergent cavities arearranged in groups, the groups of emergent cavities being uniformlydistributed over the extent of the generic substrate, each group ofcavities comprising a number of cavities.
 12. The device of claim 11,wherein the emergent cavities within each group differ in at least onecharacteristic.
 13. The device of claim 12, wherein the at least onecharacteristic comprises at least one of size, shape, or depth.
 14. Thedevice of claim 11, wherein the groups of cavities are arranged inseparate zones of the surface of the carrier.
 15. The device of claim11, wherein the cavities of each group are arranged side-by-side oneanother.
 16. The device of claim 1, wherein at least one of the emergentcavities is hermetically sealed and contains a controlled atmosphere.17. The device of claim 1, further comprising additional emergentcavities over which the surface layer has been removed.
 18. A genericsubstrate, comprising: a carrier substrate having a main face andsurfaces extending into the carrier substrate from the main face so asto define a plurality of emergent cavities extending into the carriersubstrate from the main face; and a surface layer molecularly bonded tothe main face of the carrier substrate, portions of the surface layeroverhanging the emergent cavities forming flexible membranes associatedwith the emergent cavities; wherein a shape, dimensions, and anarrangement of the emergent cavities over an extent of the genericsubstrate are not specific to one particular product or to oneparticular application, and the emergent cavities are arranged ingroups, the groups of emergent cavities being uniformly distributed overthe extent of the generic substrate, each group of cavities comprising anumber of cavities.
 19. The device of claim 18, wherein the emergentcavities within each group differ in at least one characteristic. 20.The device of claim 19, wherein the at least one characteristiccomprises at least one of size, shape, or depth.